Method and apparatus for distributed space-time coding in wireless radio networks

ABSTRACT

A method and apparatus is disclosed herein for performing distributed space-time coding. In one embodiment, the distributed space-time coding is used for downlink communications in wireless radio networks. In one embodiment, the method comprises storing information-bearing sequence at two or more base stations in a group of base stations; and transmitting data corresponding to the information-bearing sequence from a number of base stations for receipt by a receiver of a user, where the number of base stations is not globally known a priori and indicates a diversity of order, such that the diversity of order M is obtained if a total of M number of antennas spread over multiple base stations transmit the information-bearing sequence, where M is an integer.

PRIORITY

The present patent application claims priority to and incorporates byreference the corresponding patent application Ser. No. 11/754,903,entitled, “A Method and Apparatus for Distributed Space-Time Coding forthe Downlink of Wireless Radio Networks”, filed on May 29, 2007, whichclaims priority to and incorporates by reference correspondingprovisional patent application Ser. No. 60/810,457, entitled, “A Methodand Apparatus for Distributed Space-Time Coding for the Downlink ofWireless Radio Networks”, filed on Jun. 1, 2006.

FIELD OF THE INVENTION

The present invention relates to the field of space-time coding; moreparticularly, the present invention relates to distributed space-timecoding for downlink communications in a wireless radio network.

BACKGROUND OF THE INVENTION

The problem of interest arises in settings involving the downlink ofcellular systems whereby the information sequence is available atmultiple base stations. The invention exploits intelligent transmissionof the information bearing signal over the multiple independently fadingpaths from each transmitting base station to the receiver to providediversity and thus coverage/reliability benefits to the receiver. Oneembodiment of the invention draws upon connections between the givensetting, where data are available at multiple base stations, andsettings involving a single active base station with multiple transmitantennas. In particular, it builds upon the existing body of work on theuse space-time block codes (STBCs) for providing diversity in thedownlink in the case that a single base station with multiple transmitantennas is employed for transmission.

A large collection of STBCs have been proposed in recent years as a wayof providing diversity and/or multiplexing benefits by exploitingmultiple transmit antennas in the forward link of cellular systems.Given the presence of N transmit antennas, the typical objective is todesign the STBC so as to provide order-N diversity in the system.Typical STBC designs encode blocks of K symbols that are transmittedover each of the N antennas by T samples, where T is greater than orequal N, as well as K. Such STBC designs are described by a T×N STBCmatrix, whose (i,j)th entry denotes the sample transmitted by the j-thantenna at time i. Of interest are full-rate schemes, i.e., schemeswhere the effective data transmission rate R=K/T equals 1 symbol/channeluse. Another important attribute of a STBC is its decoding complexity.Although the complexity of arbitrary STBCs is exponential in the size K″of the jointly encoded symbols, there exist designs with much lowercomplexity. One such attractive class of designs, referred to asorthogonal space-time codes, can provide full diversity while theiroptimal decoding decouples to (linear processing followed by)symbol-by-symbol decoding.

In S. Yiu, R. Schober and L. Lampe, “Distributed Block Source Coding,”IEEE GLOBECOM 2005 Proceedings, November 2005, a distributed space-timecoding method is presented for providing diversity benefits in thesetting of interest. The method exploits a standard order-N diversitySTBC together with a base station specific “postcoding” operation.

Orthogonal space-time block code designs are well-known in the art andhave the following features: they provide full (order-N) diversity; theyallow symbol-by-symbol decoding; and their (column) shortened versionsare also orthogonal designs, thus providing such orthogonal designs forsystem with any number of antennas less than or equal to N.

Despite the obvious advantages of orthogonal STBC designs in adistributed downlink transmission environment, a key shortcoming withthis approach stems from its limited applicability, in the form of thelimitations it incurs in the transmission parameters. In particular, no(complex) full-rate orthogonal designs exist for more than two antennas.For more information, see H. Jafarkani, Space-Time Coding, Theory andPractice, Cambridge University Press, 2005. Specifically, the maximumtransmission rate with an OSTBC for an N>3 transmit-antenna setting isprovably upper-bounded by ¾ (H. Wang and X.-G. Xia, “Upper bounds ofrates of space-time block codes from complex orthogonal designs,” IEEETrans. Information Theory, pp. 2788-2796, October 2003). Furthermore,although ¾ rate orthogonal space-time codes have been found for N=3 andN=4 transmit antennas, it is not known whether or not the ¾ rate isachievable for N>4 with the restriction of orthogonal designs. In fact,the highest known rate achievable by systematic (complex) orthogonalSTBC designs is ½. Such designs make an inefficient use of bandwidth asthey employ only half of the available dimensions in the signal spacefor transmission.

Quasi-orthogonal space-time block code designs exploit the existence ofan orthogonal design for an N transmit-antenna system to providefull-rate designs for a 2 N antenna system. Some designs employ the base(N=2) full rate orthogonal design, referred to as the “Alamouti” scheme,to obtain a full diversity system for N=4, that allows for hierarchicaldecoding. For more information, see S. M. Alamouti, “A simpletransmitter diversity scheme for wireless communications,” IEEE JournalSelected Areas in Communications, pp. 1451-1458, October 1998. Forinstance, one design encodes two blocks of two symbols at a time overfour time slots, where the four transmit antennas are split into twogroups/pairs and each of the given two pairs of symbols are usedindependently to construct two Alamouti (N=2) space-time codes. In thefirst two time slots, each of the two antenna groups signals one of thetwo “Alamouti” codes and swap for the next two slots. Provided thesecond set of symbols is from a properly rotated constellation, thesedesigns achieve full diversity and hierarchical decoding.

The use of distributed space-time block codes based on postcoders wasintroduced in S. Yiu, R. Schober and L. Lampe, “Distributed Block SourceCoding,” IEEE GLOBECOM 2005 Proceedings, November 2005. In theirtechnique, each active base station exploits the same space-time blockfor signaling. In particular, given a T×N space-time block code designedto send blocks of K symbols over n antennae at rate K/T, and given asymbol sequence that is to be transmitted, each base station generatesfirst the common code T×N STBC matrix as if there were a single “active”base station with N transmit antennas. The signal transmitted by thesingle antenna of an “active” base station is then a linear combinationof the columns of the T×N STBC matrix. This linear combination isspecific to the particular antenna at the particular base station andcan be conveniently expressed as a “projection” of the common STBC ontoa base-station specific “steering” vector. Given a set of M_(max)single-antenna base-stations, each of which can be potentially “active”,the set of M_(max) steering vectors can be jointly optimized a priori,e.g., by use of an LMS algorithm. The resulting set of optimizedsteering vectors allows the following: given any M out of M_(max) basestations are “active” (regardless of which “M” are “active”), with Mgreater or equal to N, the distributed scheme provides order-Ndiversity, i.e., the full diversity of the original STBC code.Furthermore, via the joint optimization of the steering vectors theworst-case loss in coding-gain-distance performance with respect to thestandard STBC can be minimized.

Although the above approach has a number of advantages, it also has twomain drawbacks. First, the larger the set of potentially active steeringvectors, the higher the performance loss with respect to the standardcode performance. Hence, there is a need for approaches that arescalable, i.e., approaches that scale well with changing M. The jointoptimization of the set of potentially active steering vectors proposedin S. Yiu, R. Schober and L. Lampe, “Distributed Block Source Coding,”IEEE GLOBECOM 2005 Proceedings, November 2005, is not scalable. Secondand more important, the diversity benefits of the approach are limitedby the strength of the standard STBC code employed in the design. Thus,if the well known “Alamouti” code, is employed (designed for a twoantenna system), the system provides diversity of order at most 2 evenif M, the number of “active” cooperating transmit antennas may be muchlarger than 2.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed herein for performing distributedspace-time coding. In one embodiment, the distributed space-time codingis used for downlink communications in wireless radio networks. In oneembodiment, the method comprises storing an information-bearing sequenceat two or more base stations in a group of base stations; andtransmitting data corresponding to the information-bearing sequence froma number of base stations for receipt by a receiver of a user, where thenumber of base stations is not globally known a priori and indicates adiversity of order, such that the diversity of order M is obtained if atotal of M number of antennas spread over multiple base stationstransmit the information-bearing sequence, where M is an integer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1A is an example of a wireless communication system.

FIG. 1B is a block diagram of one embodiment of a basic encoding scheme.

FIG. 2 is a block diagram of one embodiment of a basic decoding scheme.

FIG. 3 illustrates one embodiment of a precoded block-diagonal STBCencoder.

FIG. 4 illustrates one embodiment of a precoder structure allowinghierarchical decoding with the Alamouti code as a baseline.

FIG. 5 illustrates a constellation of a typical precoded symbol.

FIG. 6 illustrates linear precoding of the symbol vector s and itsconjugate with block-diagonal combining.

FIG. 7 is a block diagram of one embodiment of an encoder that allowsfor hierarchical decoding.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A number of recently developed techniques and emerging standards suggestemploying multiple antennas at a base station to improve the reliabilityof data communication over wireless media without compromising theeffective data rate of the wireless systems. Specifically, recentadvances in wireless communications have demonstrated that the presenceof multiple antennae at a base station can be exploited to providereliability (diversity) benefits as well as throughout benefits in datatransmission from the base station to cellular users. These multiplexingand diversity benefits are inherently dependent on the number oftransmit and receive antennas in the system been deployed, as they arefundamentally limited by the multiplexing-diversity trade-offs curves.

In many emerging and future radio networks, the data for any particularcell user may be available to multiple base stations. Any such basestation with data for the user of interest is referred to herein as an“active” base station. By using space-time coding techniques over theantennas of the active base stations, each of the active base stationantennas may be viewed, for a particular user, as an element of avirtual antenna array, which can be used to provide diversity benefitsto the desired user. In one embodiment, the number and set of activebase stations vary with time, due to the time-varying quality ofwireless channels relaying the data to the base-stations, or the servingdemands of the individual stations. Furthermore, the active basestations at any time may not be globally known a priori. To compensatefor this, space-time coding techniques are used to provide diversitybenefits regardless of which set of base stations are active at anytime.

In one embodiment, a technique is used to provide uniformly optimizedperformance at the receiver, regardless of the particular set of activebase stations. The technique includes the use of a common data precoderat each potentially active base station, followed by a common (to allbase stations) standard space-time block code (STBC). A postcodingoperation then follows the STBC. In one embodiment, this postcodingoperation generates the signal to be transmitted by projecting theoutput of the STBC operation on a steering vector, which is distinct foreach base station (and for each antenna at each base station, in casethere are multiple antennas at given base station). The technique isdistributed in the sense that the transmitter/encoder employed at eachbase station is the same, regardless of which other base stations areactive. In addition, in one embodiment, the receiver does not need tohave knowledge of the set of active base stations.

As will be evident from the discussion below, advantages of thetechniques described herein include, but are not limited to, improvingbit-error rates or improved coverage or throughput when more than onebase station is active (in comparison to a baseline system exploiting asingle base station for data transmission in the forward link) andproviding maximum diversity available in the set of active base stationsgiven an arbitrary set of active transmitting base stations. In oneembodiment, this additional (and full) diversity benefit is guaranteedby encoding several space-time coding blocks at a time (at the cost ofdelay), and using the common precoder described herein at eachpotentially active base station. Furthermore, in one embodiment, thecoding gain benefits of these full diversity systems are optimized usingthe post-coders employed at all the potentially active base stations.

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Some portions of the detailed descriptions that follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; electrical, optical,acoustical or other form of propagated signals (e.g., carrier waves,infrared signals, digital signals, etc.); etc.

Overview

One embodiment of the present invention allows transmission of commoninformation via a set of potentially active transmitting base stationsto a receiver. It allows reliable transmission regardless of which setof base stations is transmitting, i.e., is potentially “active”. In oneembodiment, a method uses a given t×n space-time block code that encodesk symbols at a time, to generate, at each active base station, a largercode with a processing delay T=L-times-t, where the larger code encodesK=L-times-k symbols at a time. In one embodiment, this distributedencoding scheme provides the full diversity available in the set of“active” base stations provided the number of active base stations is atmost N=L times n.

As set forth above, an embodiment of the present invention includes acommon (to all base stations) linear precoder in combination with common(to all base stations) standard baseline space-time block code (STBC)and a set of base-station- and antenna-specific post-coding/steeringvectors for distributed communication over nonselective fading channels.In one embodiment, the linear precoder, an embodiment of a method forcombining L elementary t×n STBC blocks into a T×N STBC, and a set ofsteering vectors for providing the maximum possible diversity for anyset of active base stations are configured jointly so that maximumavailable diversity is achievable for any given set of active basestations. In one embodiment, the linear precoder, an embodiment of amethod for combining L elementary t×n STBC blocks into a T×N STBC, and aset of steering vectors for providing the maximum possible diversity forany set of active base stations are configured jointly to provide fulldiversity, good coding gains and hierarchical decoding.

Communication System and Transceiver Design

In one embodiment, a transceiver is designed for distributedcommunication of a symbol stream from multiple base stations to areceiver in a communication system. Each active base station has acommon information-bearing symbol stream s_(i) available that is to becommunicated to the receiver. In one embodiment, each “active” basestation has no prior information about which of the other base stationsare active. The information-bearing symbol stream is partitioned intocontiguous blocks of K consecutive symbols. A generic block symbol ofdimension K is labeled herein as “s”. Each such block symbol iscommunicated by each active antenna via a signal with duration equal toT time slots.

FIG. 1A is a block diagram of a wireless communication system. Referringto FIG. 1A, the wireless communication system comprises multiple basestations 101 and one or more wireless communication devices 102 (e.g.,mobile phones, handsets, computers, etc.). An information-bearingsequence that is to be sent to a wireless communication device from twoor more base stations 101 in a group of base stations is stored in thosebase stations and transmitted by those base stations for receipt by areceiver of wireless communication device. The number of base stationsthat transmit the information-bearing sequence is not globally known apriori and sets the diversity of order. In this context, the term“globally” means that in general the number and the identity of the basestations and their antennas that are actively cooperating intransmitting data to a particular user is not known to the participatingbase-stations. For instance, if each base station has a single antennathe diversity of order M (e.g., 4) is obtained if M base stations (e.g.,4) transmit data corresponding to the information-bearing sequence,where M is an integer. In general, the diversity order M is obtained ifa total of M antennas (spread over multiple base stations) transmit datacorresponding to the information-bearing sequence, where M is aninteger.

At each of the base stations transmitting data corresponding to theinformation-bearing sequence, a pre-processing module 111 pre-processesthe information-bearing sequence using pre-coding that is common to allbase stations transmitting data corresponding to the information-bearingsequence to produce precoded data, a space-time block code module 112encodes the precoded data using a space time block coding scheme commonto all base stations in the number of base stations; and a postprocessing module 113 performs base station- and antenna-specificpost-processing of data generated as a result of encoding the precodeddata.

In one embodiment, pre-processing module 111 receives a vector s of K=Lk symbols and precodes the symbol vector s. In one embodiment,pre-processing module 111 pre-processes the information-bearing sequenceby linearly precoding disjoint sub-blocks of symbol vectors into a setof precoded subvectors and creating sub-blocks from the precoded data.In one embodiment, the sub-blocks are created by re-interleavingelements of the precoded vectors into a new set of vectors. Note thatsub-block creation may be performed by another module. In yet anotherembodiment, pre-processing module 111 precodes k symbol sub-vectors ofsize L in a manner that allows hierarchical decoding. In yet anotherembodiment, pre-processing module 111 generates L precoded symbols ofsize k from the k precoded sub-vectors of size L.

In one embodiment, space-time block code module 112 encodes the precodeddata by generating an STBC matrix. In one embodiment, space-time blockcode module 112 also combines elementary precoded STBC blocks into alarger STBC matrix.

In one embodiment, post-processing module 113 performs basestation-specific post-processing of data generated as a result ofencoding the precoded data. In one embodiment, post-processing module113 uses a set of post-coding steering vectors and performs thepost-processing by projecting a STBC matrix on an individual steeringvector that is specific to each base station in the base stationstransmitting the information-bearing sequence.

The base station also includes a module (not shown) that unpacks thenecessary information from blocks into the sample stream and a module togenerate one or more waveforms that are transmitted by the transmitantennas of the “active” base stations.

FIG. 1B is a more detailed block diagram of one embodiment of anencoding/transmission process at the i-th base station (if the basestation has multiple antennas, “i” denotes the index of a specificantenna at a specific active base station). The encoding/transmissionprocess is performed by processing logic that may comprise hardware(circuitry, dedicated logic, etc.), software (such as is run on ageneral purpose computer system or a dedicated machine), or acombination of both. In one embodiment, the encoding/transmissionprocess relies on the use of a full-diversity “n” transmit-antennaorthogonal space-time block code (referred to as “baseline code” in FIG.1B) designed for encoding/communicating blocks of k symbols over t timeslots. Given a blocking factor (L), the encoding/transmission processgenerates a (full diversity) space-time block code (“induced code”) thatcan encode via an L×n (N) transmit-antenna system blocks of symbols ofsize L×k (K) over L×t (T) time slots.

Referring to FIG. 1B, in one embodiment, the encoding operations of theencoding/transmission process at the i-th base station are as follows. Alinear precoder 101 receives an input 111 consisting of a symbol vectors of size K. Symbol vector 111 represents the information to betransmitted and is assumed to be generated at earlier stages. In a pilot(channel estimation) phase, the vector s employed is assumed to be knownat the receiver. In the data transmission phase, the vector s representsa set of K symbols to be transmitted to the receiver and is thus unknownto the receiver.

In one embodiment, linear precoder 101 precodes the symbol vector sthrough a linear transformation into a K×1 precoded vector z (112). Thevector z is generated by (linearly) projecting s onto a K×K precodingmatrix (W).

A partitioning module 102 partitions the K×1 vector z into L blocks ofsize k (shown as z(1), z(2), . . . , z(L) in FIG. 1). Each of theprecoded blocks z(1), z(2), . . . , z(L) of size k is encoded via thebase code 103 _(1-L), respectively, to generate L matrices of dimensiont×n, (shown as B(1), B(2), . . . , B(L) in FIG. 1).

A combining module 104 combines the base code blocks B(1), B(2), . . . ,B(L) and uses the all-zeros t×n matrix as block constituents to generatean L×L block matrix, i.e., the induced code (referred to as B in FIG.1B). The matrix B has dimensions T×N (T=L×t, N=L×n). In its simplestnon-degenerate form, matrix B is a block diagonal matrix, whereby thei-th t×n block equals B(i).

A projection module 105 of the i-th base station (or the ith activeantenna in case multi-antenna base stations are employed) operates as apost processing module and projects matrix B on its own steering vectorof size N×1 to generate the effective transmitted sample vector(referred to as x(i) in FIG. 1B).

A demultiplex and symbol unpacking module 106 of the i-th base stationconverts its sequence of effective transmit sample vectors x(i) into asequence of scalar samples in a manner well-known in the art. Apulse-shaping and modulation module 107 receive these samples andperform pulse shaping and modulation to create, in a manner well-knownin the art, the information sequence that is represented as transmitwaveform 113, which is to be transmitted over the antenna of the i-thbase station.

Thus, in one embodiment, techniques described herein improve theperformance of existing distributed space time coding schemes by jointprecoding of blocks of symbols prior to STBC, followed by postcoding, inan effort to increase the maximum diversity available by the originalSTBC. The method encodes data in blocks larger than the memory of thecode, thereby increasing the dimensionality of the signal space, andallowing diversity benefits that exceed those of the standard STBC orother prior art schemes. To achieve the full diversity available with agiven set of “active” base stations, blocks of data symbols are precodedprior to STBC encoding. In one embodiment, the precoder allows achievingfull diversity, while allowing for low-complexity optimal decoding(though not required). In one embodiment, the technique is directlycompatible with prior art postcoding techniques. Furthermore, given Mactive base stations, the technique provides diversity of order Mregardless of which set of M base-stations are “active”, and allows forjoint optimization of the postcoder with the precoder and the STBCmodules for also alleviating the worst-case coding gains over allsubsets of M-“active” base stations.

FIG. 2 is a generic block diagram of one embodiment of a receiver. Thereceiver may be a portion of wireless device 102 in FIG. 1A. Thereceiver receives waveforms from multiple based stations in thecommunication system that are transmitting data corresponding to theinformation-bearing sequence. The number of base stations is notglobally known a priori and indicates a diversity of order, such thatthe diversity of order M is obtained if a total of M number of antennas(spread over multiple base stations) transmit the information-bearingsequence, where M is an integer.

At the receiver, the transmissions (constructively) overlap with eachother, and the receiver observes the aggregate effect. The receiver doesnot need to specifically know which of the additional base-stations areassisting in the transmission. The receiver structure is the samewhether there is one or more transmitting base stations. Note that inthe channel estimation phase, the channel estimates obtained at thereceiver are the “effective” channel estimates summarizing the aggregatechannel effect through the joint transmission.

Referring to FIG. 2, during a channel estimation phase, the receiverestimates channel coefficients 211 via a straightforward application ofa standard pilot-assisted method for STBCs that is well-known in theart.

In the data detection phase, front-end filtering and demodulation module201 receives, as an input, receive waveform 210 and performs filteringand demodulation to create a complex-valued received signal sequence. Avectorization module 202 converts the received signal sequence into asequence of T-dimensional vectors. In one embodiment, given a receivedT-dimensional vector and a channel estimate vector 211, minimum distancedecoder 203 finds the induced code, which, when passed through thechannel with a given channel estimate vector is the closest in Euclideandistance to the received T dimensional vector. In many cases ofpractical interest, the complexity of this detection can be greatlyreduced, such as when the induced code has block-diagonal form.

The effective channel response seen at the receiver captures(indirectly) the number of participating base stations and theirindividual postcoding vectors in the form of “aggregate” effectivechannel coefficients. These are estimated in the channel estimationphase; the same pilot (known to the receiver) symbols are sent by allbase-stations, and the received signal is used to estimate these“aggregate-effect” channel coefficients. The minimum distance decoder inFIG. 2 (module 203) also incorporates the effect of the linear precoderin its distance metric calculations.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

There are many possible transcoder design options that can affect theperformance of the encoding and/or decoding systems described herein.For example, the choice of the precoder employed in conjunction with agiven baseline code and a given blocking factor L has an impact on thediversity and coding gain distance properties of the induced code aswell as the decoder complexity. One embodiment of a precoder isdescribed below.

Precoder-Postcoder Embodiments

In one embodiment, the function of the precoder is to reshape the errorconstellations of the induced code (which is an L-fold extension of theunderlying baseline code), thereby improving the diversity/coding gainprofile of the induced code. In one embodiment, these performancemetrics are improved by implementing a precoder with the followingcharacteristics.

First, the baseline code used is the well-known Alamouti code (S. M.Alamouti, “A Simple Transmitter Diversity Scheme for WirelessCommunications,” IEEE Journal Selected Areas in Communications, pp.1451-1458, October 1998, incorporated herein by reference). The Alamouticode is an orthogonal STBC designed for n=2 transmit antennas, and codesk=2 symbols over t=2 time slots, where the blocking factor is selectedas L=2. Thus, using this code, 4 symbols are encoded at a time (s₁, s₂,s₃, and s₄) over 4 time slots and 4 antennas. The induced code B isselected as a block-diagonal matrix with the two diagonal blocksselected as the two base-codes B(1) and B(2) generated by the precodingvectors z(1) and z(2).

Second, assuming no precoding is used (i.e., W is the identity matrix inFIG. 4), the system provides diversity of order 2. In particular,diversity equals the smallest count, among all possible error events, ofthe eigenvalues of the code-error matrix that are different from zero.In the example, there are 4 eigenvalues (referred to herein as λ(m), form=1, 2, 3, 4). Due to the assumed block diagonal structure of B, bothλ(1) and λ(2), equal|e₁(1)|²+|e₂(1)|²,where e₁(1) and e₂(1) are the error events on the first and second entryof z(1) (corresponding in this case to the error events in s₁ and s₂,respectively). Similarly, both λ(3) and λ(4), equal|e₁(2)|²+|e₂(2)|²,where e₁(2) and e₂(2) are the error events on the first and second entryof z(2) (corresponding in this case to the error events in s₃, s₄,respectively). The order-2 diversity in this case arises from the factthat there are nonzero error events that make two of the eigenvaluesequal to zero (e.g. error events where there is no error in s₁ and s₂,but there is in at least one of s₃, s₄, or vice-versa).

Third, by properly precoding linear combinations of the original symbolsin each of the entries of z, diversity of order-4 can be guaranteed withvery good coding gains. One such design is shown in FIG. 4. In thisdesign, the first element of z(1) and z(2) are encoded as a linearcombination of the first two symbols s₁ and s₂, while their secondelement are a linear combination of s₃ and s₄. The complex scalingconstant properly are selected so that the error constellations ofe₁(1), e₂(1), e₁(2) and e₂(2) can be pre-shaped, so that any nonzeroerror pattern in the original set s results in all four eigenvalues ofthe code-error matrix to be nonzero, thereby guaranteeing diversity oforder 4. The resulting constellations of the elements of z are shown inFIG. 5, for the case that each symbol s_(m) is from a QPSKconstellation, and resemble superposition codes.

Fourth, if the antennas in the four-antenna system representsingle-antenna elements at four distinct base stations, and if M(single-antenna) base stations out of 4 of these base stations areactive, the proposed design guarantees full-rate transmission anddiversity of order M, which is advantageous.

Note that by restricting the precoder to be of the form of FIG. 4,precoded symbols are formed as linear combinations of only 2 symbols ata time, which is suboptimal. Brute force optimization of the precoder inthis case shows that the restrictions of the design in FIG. 4 yield anegligibly small cost in coding gain. More important, the precoder formof FIG. 4 allows for hierarchical decoding (decoding 2 symbols at a timeat the receiver).

Thus, as described above, linear precoding of K dimensional symbol(i.e., a total of L×k-dimensional symbols) is performed, through anappropriately designed linear precoder to produce L×k-dimensionalprecoded symbol vectors, where each of these L vectors is encoded withthe baseline code to generate a t×n STBC matrix. In the simplest form ofthe method, the T slots are split over L contiguous sets of t slots, andover each t slot interval one of the codes is transmitted from one of Lsize-N set of potentially active base stations.

Furthermore, the techniques described herein allow for systematicallyextending the original orthogonal STBC designs to obtain communicationssystems where the total number of antennas is more than twice the numberof antennas in the originally employed space-time code.

Precoding and Hierarchical Decoding

Pre-shaping the error constellation through linear precoding naturallyextends to the more general setting involving L factors greater than 2,as well as t, k, and n dimensions exceeding 2. In one embodiment, asystematic precoding approach for obtaining full diversity transmissionregardless of the set of active base-stations, in the case that the basecode satisfies k≦t, while allowing for hierarchical decoding. FIG. 7 isa block diagram of one embodiment of an encoder that illustrates such anapproach. The operations of FIG. 7 are performed by processing logicthat may comprise hardware (circuitry, dedicated logic, etc.), software(such as is run on a general purpose computer system or a dedicatedmachine), or a combination of both.

Referring to FIG. 7, a partition module 701 receives a symbol vector sof dimension K=L×k and split s into k symbol sub-vectors of size L(shown as s(1), s(2), . . . , s(k)). A set of precoders 702 _(1-k)precodes each sub-vector via the same linear L×L precoder Wo to obtain kprecoded symbols of dimension L×1, referred to as v(1), v(2), . . . ,v(k). An interleave module 703 constructs the L k×1 precoded symbolsz(1), z(2), . . . , z(L). In one embodiment, interleave module 703operates as follows: the entries of z(1) are the first elements of v(1),v(2), . . . , v(k); the entries of z(2) are the second elements of v(1),v(2), . . . , v(k); the construction continues in the same fashion untilthe entries of z(L) are set as the last elements of v(1), v(2), . . . ,v(k).

An encoder 704 encodes each of the precoded blocks z(1), z(2), . . . ,z(L) of size k via the base code 704 _(1-L) respectively, to generate Lmatrices of dimension t×n (shown as B(1), B(2), . . . , B(L) in FIG. 7,which is a block-diagonal B).

A projection module 705 of the i-th base station projects matrix B onits own steering vector of size N×1 to generate the effectivetransmitted sample vector. More specifically, each of baseline codesB(1), B(2), . . . , B(L) are projected onto a separate subvector of thei-th steering vector.

A demultiplex and symbol unpacking module 706 of the i-th base stationconverts its sequence of effective transmit sample vectors x(i) into asequence of scalar samples. A pulse-shaping and modulation module 707receive these samples and perform pulse shaping and modulation to createthe information sequence that is represented as transmit waveform 713,which is to be transmitted over the antenna of the i-th base station.

Also as described above, techniques described herein rely on an STBCdesigned to encoded k symbols at a time over t time slots and nantennas. In this case, each base station encodes L blocks of k symbolsat a time, incurring a delay of L×T slots. Prior to forming the STBC,the data symbols are precoded by using a linear transformation. Theeffect of the linear transformation is to spread each symbol over the Lcoding blocks and allow a diversity order of up to L×n, even though thestandard STBC can only provide diversity of order n on its own. In oneembodiment, time-varying steering vectors are also used for furtheroptimizing the coding gains of the overall system.

Performance Enhancements

There is at least two more performance enhancements that may be used.First, the use of a linear precoder operating on the both the complexsymbol vector and its conjugate. Such a precoder structure is shown inFIG. 6, where both W and V are in the form amenable to theimplementation in FIG. 7 (where each of the v(m) vectors is constructedas the sum of precoding s(m) via Wo and its conjugate via an L×Lprecoding matrix Vo associated with V). This enhancement provides morefreedom in designing the precoder system, and can yield improvements inthe coding gain distance of the induced code, while still being amenableto hierarchical decoding. Second, the block diagonal structure of thecombining of the individual blocks can be replaced in many cases (suchas the design in FIG. 4) by one where all the blocks in the m-th blockcolumn of B are equal to B(m). This construction has betterpeak-to-average power ratio than the block diagonal ones, and can stillallow hierarchical decoding.

Examples of Advantages of Embodiments of Invention

There are a number of advantages associated with embodiments describedherein. First, embodiments described herein allow opportunisticdiversity/reliability improvements in wireless communication of data toa receiver, by exploiting the availability of the same data andsignaling resources at other active base stations. This reliabilityimprovement comes at no cost in total transmit power per symbol, datarate, or bandwidth. Second, given an arbitrary set of active basestations, techniques can provide full diversity benefits regardless ofwhich set of the base stations are in the active set. Third, theproposed technique is distributed, i.e., the encoding at any active basestation is performed without knowledge of which of the other basestations are active. Fourth, certain embodiments of the disclosed methodallow low-complexity decoding at the receiver. Lastly, in oneembodiment, the average received signal SNR can be taken into account toaccommodate the fact that distinct active base stations can be atdifferent distances from the receiver, and thus the average receivedsignal strengths differ.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

I claim:
 1. A method for sending an information-bearing sequence to areceiver through cooperation of an arbitrary set of base stations, themethod comprising a series of operations performed at each base stationin the arbitrary set of base stations, the series of operationscomprising: dividing data into blocks; and sequentially andindependently encoding the blocks by precoding each data block toproduce precoded data using a precoder that is common to base stationsin the arbitrary set of base stations, wherein precoding each data blockto produce precoded data comprises linearly precoding disjointsub-blocks of symbol vectors into a set of precoded subvectors, creatingsub-blocks from the precoded data, wherein creating sub-blocks from theprecoded data comprises re-interleaving elements of the precoded vectorsinto a new set of vectors, encoding the sub-blocks independently using aspace-time block code (STBC) that is common to base stations in thearbitrary set of base stations to generate an STBC matrix, andperforming base station-specific and antenna-specific post-coding byprojecting the STBC matrix on an individual steering vector that isspecific to said each base station.
 2. The method defined in claim 1wherein the precoder, the STBC and the post coding, in combination,provide full diversity.
 3. The method defined in claim 1 whereinlinearly precoding disjoint sub-blocks of symbol vectors into a set ofprecoded subvectors further comprises linearly precoding disjointsub-blocks of symbol vectors and their conjugates into the set ofprecoded subvectors.
 4. The method defined in claim 1 further comprisingoutputting distinct waveforms for transmission by each transmit antennaof said each base station.
 5. The method defined in claim 4 furthercomprising: demultiplexing and unpacking symbols of a transmit samplevector to create unpacked data; and performing pulse-shaping andmodulation on the unpacked data to create the at least one waveform. 6.The method defined in claim 1 further comprising combining precoded STBCblocks into a STBC matrix.
 7. A base station for use in a communicationsystem that sends an information-bearing sequence to a receiver throughcooperation of an arbitrary set of base stations, each base station inthe arbitrary set of base stations comprising: a number of antennaelements; a processing unit to divide data into blocks and sequentiallyand independently encode the blocks, wherein the processing unitcomprises a precoder to precode each data block to produce precoded dataand create sub-blocks from the precoded data, the precoder being commonto base stations in the arbitrary set of base stations, wherein theprecoder to precode each data block to produce precoded data comprisesthe precoder to linearly precode disjoint sub-blocks of symbol vectorsinto a set of precoded subvectors, and the precoder to create sub-blocksfrom the precoded data further comprises the precoder to re-interleaveelements of the precoded vectors into a new set of vectors, a space-timeblock code module to encode sub-blocks of the precoded dataindependently using a space-time block code (STBC) that is common tobase stations in the arbitrary set of base stations, the space-timeblock code module operable to generate an STBC matrix, and a post codingmodule to perform base station specific post coding by projecting theSTBC matrix on an individual steering vector that is specific to saideach base station and each antenna element to said each base station;and a transmitter to transmit a waveform that is generated based on dataresulting from performing the base-station-specific and antenna-specificpost coding.